Photoexcitable material, photochemical electrode, and method for manufacturing photoexcitable material

ABSTRACT

A photoexcitable material includes: a solid solution of MN (where M is at least one of gallium, aluminum and indium) and ZnO, wherein the photoexcitable material includes 30 to 70 mol % ZnO and has a band gap energy of 2.20 eV or less.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-108760, filed on May 31,2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to photoexcitablematerials, photochemical electrodes, and methods for manufacturingphotoexcitable materials.

BACKGROUND

Technologies that utilize solar energy include artificial photosynthesisand photocatalysis.

Examples of the related art are disclosed in Japanese Laid-open PatentPublication Nos. 2005-144210, 2006-116415, and 2012-187520; Hashiguchi,H. et al., “Photoresponse of GaN:ZnO Electrode on FTO under VisibleLight Irradiation”, Bull. Chem. Soc. Jpn., 82, 401-407 (2009); andJensen, L. L., Muckerman, J. T. & Newton, M. D., “First-PrinciplesStudies of the Structural and Electronic Properties of the(Ga_(1-x)Zn_(x))(N_(1-x)O_(x)) Solid Solution Photocatalyst”, J. Phys.Chem. C, 112, 3439-3446 (2008).

SUMMARY

According to an aspect of the embodiments, a photoexcitable materialincludes: a solid solution of MN (where M is at least one of gallium,aluminum and indium) and ZnO, wherein the photoexcitable materialincludes 30 to 70 mol % ZnO and has a band gap energy of 2.20 eV orless.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example sectional view of a photochemical electrode;

FIG. 2 illustrates example X-ray diffraction spectra of solid solutionpowders; and

FIG. 3 illustrates example relationships between composition and bandgap energy of Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x).

DESCRIPTION OF EMBODIMENTS

As an example, artificial photosynthesis produces hydrogen gas fromwater and synthesizes an organic substance from water and carbon dioxidegas. As another example, photocatalysis decomposes contaminants. Forexample, these technologies use photoexcitable materials.

Photoexcitable materials are semiconductors, which have a forbidden bandbetween the valence band and the conduction band. Photoexcitablematerials absorb sunlight to excite electrons from the valence band tothe conduction band, leaving holes in the valence band. The resultingexcited electrons and holes reduce or oxidize water or contaminants. Toenhance the utilization of solar energy, a photoexcitable material maybe provided that absorbs the solar spectrum from short wavelengths to aslong a wavelength as possible. To provide such a photoexcitablematerial, a narrower forbidden band is desirable.

Gallium nitride (GaN) and zinc oxide (ZnO) are UV-responsivephotoexcitable materials having forbidden band widths of about 3.1 eVand about 3.2 eV, respectively. GaN and ZnO both have a Wurtzite-typecrystal structure and are mixed in any ratio to form aGa_(x)N_(x)Zn_(1-x)O_(1-x) solid solution having the same structure.

For example, the band gap energy of Ga_(1-x)N_(1-x)Zn_(x)O_(x) solidsolutions containing 30% or less ZnO that are synthesized by solid-statereactions decreases with increasing x. At x=0.3, the band gap energy is2.5 eV.

For example, simulations for Ga_(1-x)N_(1-x)Zn_(x)O_(x) solid solutionsby first-principles calculations reveal a downward-bowing compositiondependence of band gap energy, with the minimum reached when the ZnOcontent is around 60%. Extrapolation of these simulation results toexperimental data yields a minimum band gap energy of 2.29 eV atx=0.525.

Ga_(1-x)N_(1-x)Zn_(x)O_(x) solid solutions have narrower forbidden bandsthan pure GaN and ZnO. This is because (N 2p)-(Zn 4s, 4p) bonding occursnear the top of the valence band of Ga_(1-x)N_(1-x)Zn_(x)O_(x) solidsolutions newly. A narrower forbidden band results in a higherefficiency of solar energy utilization. For example, simply preparing asolid solution of GaN and ZnO may result in a limited reduction inforbidden band width. Accordingly, there may be provided anarrow-forbidden-band photoexcitable material that allows for efficientutilization of light energy, a photochemical electrode including such aphotoexcitable material, and a method for manufacturing such aphotoexcitable material.

A photoexcitable material may be a solid solution of MN (where M is atleast one of gallium, aluminum and indium) and ZnO. The photoexcitablematerial may contain ZnO in an amount of 30 to 70 mol %, more than 30 to70 mol %, or 40 to 60 mol %. A ZnO content of less than 30 mol % or morethan 70 mol % may fail to provide a band gap energy of 2.20 eV or less.The photoexcitable material may have a band gap energy of 2.20 eV orless, 1.90 to 2.20 eV, 1.90 to 2.10 eV, or 1.95 to 2.00 eV. Gallium,aluminum, and indium share the common feature of being a group 13element. The term “photoexcitable material” refers to a material that isexcited upon absorbing light.

The photoexcitable material may be represented by formula (1):

M_(1.00-x)N_(1.00-x)Zn_(x)O_(x)  formula (1)

where 0.30≦x≦0.70, 0.30<x≦0.70, or 0.40≦x≦0.60 may be satisfied.

The photoexcitable material may be layered. An average thickness of thelayer of the layered photoexcitable material has no limit specially andmay be selected suitably depending on the purpose. For example, thelayer of the photoexcitable material may have an average thickness of0.5 to 5 μm.

GaN—ZnO-based materials are solid solutions having a hexagonalWurtzite-type crystal structure. The composition of these solidsolutions varies continuously as gallium and zinc are replaced with eachother and nitrogen and oxygen are replaced with each other. GaN—ZnOsolid solutions have a narrower band gap than simple GaN material andsimple ZnO material. As more ZnO is dissolved in GaN, the end ofabsorption wavelengths may extend to longer wavelengths.

For example, the forbidden band of Ga_(1-x)N_(1-x)Zn_(x)O_(x) solidsolutions is narrowest when x is about 0.5, at which the forbidden bandwidth (band gap energy) is about 2.5 eV. In the solar spectrum,radiation with photon energies of 2.5 eV or more accounts for only 20%of the total energy radiation. Accordingly, the efficiency of solarenergy utilization may be enhanced by further reducing the forbiddenband width of Ga_(1-x)N_(1-x)Zn_(x)O_(x).

For example, a solid solution of MN (where M is at least one of gallium,aluminum and indium) and ZnO may be deposited by nanoparticle deposition(NPD) to obtain a photoexcitable material having a very small band gapenergy. For example, the NPD process disclosed in Documents 1 to 3 belowmay be used. NPD allows a film of a highly crystalline ceramicnanoparticle assembly to be formed at low temperatures without the useof resin components.

Document 1: Imanaka, Y., Amada, H. & Kumasaka, F., “Dielectric andInsulating Properties of Embedded Capacitor for Flexible ElectronicsPrepared by Aerosol-Type Nanoparticle Deposition”, Jpn. J. Appl. Phys.,52, 05DA02 (2013)

Document 2: Imanaka, Y. et al., “Nanoparticulated Dense and Stress-FreeCeramic Thick Film for Material Integration”, Adv. Eng. Mater., 15,1129-1135 (2013)

Document 3: Imanaka, Y., Amada, H., Kumasaka, F., Awaji, N. & Kumamoto,A., “Nanoparticulate BaTiO₃ Film Produced by Aerosol-Type NanoparticleDeposition”, J. Nanopart. Res., 18, 102 (2016)

For example, the band gap energy may be 2.5 eV. For example, the bandgap energy may be 2.29 eV. For example, there may be provided anM_(1.00-x)N_(1.00-x)Zn_(x)O_(x) solid solution having a band gap energyof 2.20 eV or less and a method for preparing such a solid solution.

A photoexcitable material may be prepared by NPD as follows.

NPD may be performed by the process disclosed in Documents 1 to 3. AnNPD process involves ejecting an aerosol in which the particles aredispersed in a gas from a nozzle onto a substrate, the aerosol collidesagainst the substrate and a film including the feedstock particles isformed on the substrate.

An inorganic film is formed by NPD. In a system continuously evacuatedwith a vacuum pump, inorganic feedstock particles form an aerosol with agas stream and are carried. The carried feedstock particles collide witheach other in the nozzle and are crushed. In the feedstock particles,the crystallinity of the interior of the feedstock particles ismaintained while part of the crystal structure of the surface of thefeedstock particles is distorted, and, thus, the surface energy level ofthe feedstock particles increases. When the crushed feedstock particlesare deposited on the substrate, the crushed feedstock particles arerecombined together by an action (cohesive force) in which thehigh-energy-level, unstable surface of the feedstock particles isstabilized. As a result, a dense film of a nanoparticle assembly isformed on the substrate at room temperature. If the film is annealed,the optimum sintering temperature is reduced by at least 500° C. sincethe interior of the film is composed of nanoparticles.

For example, if a GaN—ZnO-based material is deposited by NPD at a highparticle flow velocity, the resulting film has a significantly smallerband gap energy than the GaN—ZnO-based materials corresponding tofeedstock.

The GaN—ZnO-based materials corresponding to feedstock particles(Ga_(1.00)N_(1.00-x)Zn_(x)O_(x) solid solution) are prepared, forexample, by a solid-state reaction of oxide feedstocks (Ga₂O₃ and ZnO)at an elevated temperature, for example, 1,123 K (about 850° C.), in anammonia gas stream. This method of preparing the feedstock particles mayinvolve the following phenomenon. When Ga₂O₃ and ZnO react to form asolid solution, ZnGa₂O₄ forms readily as a by-product. Once ZnGa₂O₄forms, a reaction of the GaN—ZnO-based material may not proceed evenwhen ZnGa₂O₄ is made react in an ammonia gas stream.

Accordingly, for example, in a method for manufacturing feedstockparticles (GaN—ZnO-based material (Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x)solid solution)), serving as a photoexcitable material, by reacting amixture of GaN and ZnO, a generation of impurities such as ZnGa₂O₄ isreduced and a solid solution of GaN and ZnO is obtained.

A method for manufacturing a photoexcitable material includes heating amixture of MN (where M is at least one of gallium, aluminum and indium)and ZnO to form a solid solution of MN and ZnO.

The photoexcitable material may be used as feedstock particles for themanufacture of a photoexcitable material having a band gap energy of2.20 eV or less.

A heating temperature for heating the mixture has no limit, may beselected suitably depending on the purpose, may be, for example, 500° C.to 900° C., and may be, for example, 600° C. to 850° C. When the heatingtemperature is within a preferred range, unreacted residues and thegeneration of impurities may be reduced more effectively.

The mixture may be heated in an inert atmosphere or ammonia atmosphere.The inert atmosphere may be, for example, argon or nitrogen.

An amount of ZnO regarding the solid solution has no limit and isselected suitably depending on the purpose. For example, a solidsolution containing 30 to 70 mol % ZnO may be used as feedstockparticles for the manufacture of a photoexcitable material having a bandgap energy of 2.20 eV or less.

A mixing proportion of MN and ZnO in the mixture has no limit and may beselected suitably depending on the purpose. For example, ZnO, which mayvolatilize during heating, may be present in an amount larger than theproportion of ZnO to MN in the solid solution to obtain a solid solutionhaving the desired composition.

A photochemical electrode includes at least a conductive layer and alayered photoexcitable material (which may hereinafter be referred to as“photoexcitable material layer”) and may optionally include othercomponents.

The conductive layer may be a layer with conductivity and a shape, asize and a structure of the conductive layer have no limit and may beselected suitably depending on the purpose. Examples of materials forthe conductive layer include metals and metal oxides. Examples of metalsinclude silver (Ag), gold (Au), copper (Cu), platinum (Pt), palladium(Pd), tungsten (W), nickel (Ni), tantalum (Ta), bismuth (Bi), lead (Pb),indium (In), tin (Sn), zinc (Zn), titanium (Ti), and aluminum (Al).Examples of metal oxides include tin-doped indium oxide (ITO),fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), zincoxide, indium oxide (In₂O₃), aluminum-doped zinc oxide (AZO),gallium-doped zinc oxide (GZO), tin oxide, zinc oxide-tin oxide-basedmaterials, indium oxide-tin oxide-based materials, and zinc oxide-indiumoxide-magnesium oxide-based materials.

If the conductive layer is a thin film, it may be supported on asupport. The support may be, for example, a glass substrate.

The photoexcitable material layer is a layered photoexcitable material.For example, as described above, the photoexcitable material layer maybe formed by NPD.

An average thickness of the photoexcitable material layer has no limitand may be selected suitably depending on the purpose, for example, 0.5to 5 μm.

FIG. 1 is an example sectional view of a photochemical electrode. Thephotochemical electrode in FIG. 1 includes a conductive layer 1 and aphotoexcitable material layer 2 on the conductive layer 1. Theconductive layer 1 may be supported on a support such as a glasssubstrate.

The photochemical electrode may be used as an anode for light-dependentreactions of artificial photosynthesis. A carbon dioxide reductionapparatus used for artificial photosynthesis includes, in sequence, thephotochemical electrode, serving as an anode, a proton-permeablemembrane, and a cathode and may optionally include other components.

In band gap energy measurements and evaluations, reflectance is measuredfor each wavelength using a UV-visible spectrophotometer (V-650)available from JASCO Corporation. After the measurement, the band gapenergy is calculated by the following procedure. The measuredreflectance is substituted into Kubelka-Munk function (1):

$\begin{matrix}{{F\left( R_{\infty} \right)} = \frac{\left( {1 - R_{\infty}} \right)^{2}}{2\; R_{\infty}}} & (1)\end{matrix}$

where

-   -   F(R_(∞)): Kubelka-Munk function    -   R_(∞): absolute reflectance

A graph of (hνα)̂(1/n) on the vertical axis against energy (hν) on thehorizontal axis is then obtained from equation (2) below. Theintersection of the gradient (gradient at an inflection point of thegraph) and the baseline is calculated. The band gap energy is defined asthe value at the intersection on the horizontal axis.

$\begin{matrix}{\left( {{hv}\; \alpha} \right)^{\frac{1}{n}} = {A\left( {{hv} - {Eg}} \right)}} & (2)\end{matrix}$

where

-   -   h: Planck constant    -   ν: frequency    -   hν=1239.7/λ (λ: wavelength)    -   α: absorption coefficient (replaced with F(R_(∞)))    -   Eg: band gap energy    -   A: proportionality constant    -   n: ½ for direct transition and 2 for indirect transition

A three-electrode electrochemical cell is used for measurements. Theworking electrode is a specimen (having an area of 0.75 cm²) depositedon an FTO substrate. The counter electrode is a platinum foil. Thereference electrode is a Ag/AgCl (saturated aqueous KCl solution)electrode. The electrolyte is a 0.5 M Na₂SO₄ aqueous solution (pH=6.0 to6.5) which is bubbled with nitrogen gas for 30 minutes in advance toremove dissolved oxygen. The potential is corrected, when appropriate,with a reference value of the normal hydrogen electrode at pH=0 (vs.NHE), and the corrected potential is presented. A photocurrentdensity-potential curve is measured by sweeping the potential from −0.4V to 1.6 V while intermittently exposing the specimen to AM 1.5Gartificial sunlight (at 100 mW/cm²).

Manufacturing Example 1

GaN and ZnO are mixed such that the molar ratio of GaN to ZnO in theresulting solid solution is 55:45. The mixture is reacted at 1,123 K(about 850° C.) in an ammonia gas stream for 12 hours to obtain aGa_(0.55)N_(0.55)Zn_(0.45)O_(0.45) solid solution powder having anaverage particle size of 1 μm.

Manufacturing Example 2

Ga₂O₃ and ZnO are mixed such that the molar ratio of GaN to ZnO in theresulting solid solution is 55:45. The mixture is reacted at 1,123 K(about 850° C.) in an ammonia gas stream for 12 hours to obtain aGa_(0.55)N_(0.55)Zn_(0.45)O_(0.45) solid solution powder having anaverage particle size of 1 μm.

FIG. 2 illustrates example X-ray diffraction spectra of the solidsolution powders. The solid solution powders obtained in ManufacturingExamples 1 and 2 are analyzed by X-ray diffraction. The results areillustrated in FIG. 2. FIG. 2 depicts diffraction peaks for simple GaN,simple ZnO, and ZnGa₂O₄. The solid solution powder obtained in Example 2exhibits diffraction peaks attributed to GaN and ZnO (indicated by thecircles) and diffraction peaks attributed to ZnGa₂O₄ (indicated by theinverted triangles). The solid solution powder obtained in Example 1exhibits diffraction peaks attributed to GaN and ZnO (indicated by thecircles) but no diffraction peaks attributed to ZnGa₂O₄. Thus, themethod of manufacture in Manufacturing Example 1 produces smalleramounts of impurities than the method of manufacture in ManufacturingExample 2.

Manufacturing Example 3

GaN and ZnO are mixed in predetermined amounts. The mixtures are reactedat 1,123 K (about 850° C.) in an ammonia gas stream to obtainGa_(1.00-x)N_(1.00-x)Zn_(x)O_(x) feedstock powders having a purity of99.9% or more and an average particle size of 1 μm. Table 1 lists thecompositions and band gap energies of the resulting feedstock powders(Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x)).

TABLE 1 Composition formula x Band gap energy (eV) GaN 0.00 3.04Ga_(0.90)N_(0.90)Zn_(0.10)O_(0.10) 0.10 2.81Ga_(0.76)N_(0.76)Zn_(0.24)O_(0.24) 0.24 2.59Ga_(0.69)N_(0.69)Zn_(0.31)O_(0.31) 0.31 2.54Ga_(0.55)N_(0.55)Zn_(0.45)O_(0.45) 0.45 2.53 ZnO 1.00 3.23

The nanoparticle deposition (NPD) apparatus used includes an aerosolgenerator system, a deposition chamber, and a vacuum system. Thedeposition unit may include no heat source. EachGa_(1.00-x)N_(1.00-x)Zn_(x)O_(x) feedstock powder manufactured inManufacturing Example 3 and having a purity of 99.9% or more and anaverage particle size of 1 μm is placed in a vessel of the aerosolgenerator system and is subjected to vibrations at 10 Hz. A helium gashaving a pressure of 0.2 MPa and a purity of 99.9% is then introducedinto the vessel to generate an aerosol. The resulting aerosol is fed toa nozzle in the deposition chamber. The internal pressure of thedeposition chamber is controlled to less than 10 Pa by a mechanicalbooster and a vacuum pump. The aerosol is ejected from the nozzle andcollides onto an FTO substrate (glass substrate having a fluorine-dopedtin oxide thin film formed thereon) placed in the deposition chamber for10 minutes. The gas flow velocity during this process is 50 to 100m/sec. The gas flow velocity is calculated from the flow rate of the gaspassing through the nozzle orifice. As a result, aGa_(1-x)N_(1-x)Zn_(x)O_(x) photoexcitable material layer (having anaverage thickness of 3 μm) is formed on the FTO substrate at roomtemperature. After the deposition, the photoexcitable material layer isannealed at 600° C. in a nitrogen atmosphere for 30 minutes to restorecrystallinity. In this way, photoexcitable material layers are formed onFTO substrates to obtain photochemical electrodes.

The band gap energies of the photoexcitable material layers of theresulting photochemical electrodes are determined, as listed in Table 2.

TABLE 2 Composition formula x Band gap energy (eV) GaN 0.00 2.92Ga_(0.76)N_(0.76)Zn_(0.24)O_(0.24) 0.24 2.35Ga_(0.69)N_(0.69)Zn_(0.31)O_(0.31) 0.31 2.18Ga_(0.55)N_(0.55)Zn_(0.45)O_(0.45) 0.45 1.95 ZnO 1.00 3.06

Of the resulting photochemical electrodes, the photochemical electrodemanufactured using a Ga_(0.55)N_(0.55)Zn_(0.45)O_(0.45) solid solutionas a feedstock powder is tested for photocurrent. This photochemicalelectrode is found to produce a very large photocurrent, for example,600 μA/cm².

FIG. 3 illustrates example relationships between composition and bandgap energy of Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x). FIG. 3 depicts the bandgap energies of the Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x) solid solutionsobtained in Manufacturing Example 3 and the band gap energies of thephotoexcitable material layers of the photochemical electrodes listed inTable 2. Quadratic polynomial fitting curves obtained from these resultsare added to FIG. 3. A band gap energy in a photoexcitable materiallayer (Ga_(1.00-x)N_(1.00-x)Zn_(x)O_(x) solid solution) of aphotochemical electrode is 2.20 eV or less in a range of 30 to 70 mol %of ZnO content and light energy may be utilized with high efficiency.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A photoexcitable material comprising: a solidsolution of MN (where M is at least one of gallium, aluminum and indium)and ZnO, wherein the photoexcitable material includes 30 to 70 mol % ZnOand has a band gap energy of 2.20 eV or less.
 2. The photoexcitablematerial according to claim 1, wherein the photoexcitable material isrepresented by the following formula:M_(1.00-x)N_(1.00-x)Zn_(x)O_(x) where 0.30≦x≦0.70.
 3. The photoexcitablematerial according to claim 1, wherein M is gallium.
 4. Thephotoexcitable material according to claim 1, wherein the photoexcitableis layered.
 5. A photochemical electrode comprising: a conductive layer;and a photoexcitable material provided over the conductive layer andbeing layered, wherein the photoexcitable material includes a solidsolution of MN (where M is at least one of gallium, aluminum andindium), includes 30 to 70 mol % ZnO and has a band gap energy of 2.20eV or less.
 6. The photochemical electrode according to claim 5, whereinthe photoexcitable material is represented by the following formula:M_(1.00-x)N_(1.00-x)Zn_(x)O_(x) where 0.30≦x≦0.70.
 7. The photochemicalelectrode according to claim 5, wherein M is gallium.
 8. Thephotochemical electrode according to claim 5, wherein the photoexcitableis layered.
 9. A method for manufacturing a photoexcitable material,comprising: preparing an aerosol in which particles of a solid solutionof MN (where M is at least one of gallium, aluminum and indium) and ZnOare dispersed in a gas; and forming a photoexcitable material layerincluding the particles on a substrate or a conductive layer by ejectingthe aerosol from a nozzle onto the substrate or the conductive layer insuch a manner the particles collide against a surface of the substrateor the conductive layer.
 10. The method according to claim 9, whereinthe photoexcitable material contains 30 to 70 mol % ZnO and has a bandgap energy of 2.20 eV or less.
 11. The method according to claim 9,further comprising annealing the photoexcitable material layer.
 12. Themethod according to claim 11, wherein the annealing comprises annealingthe photoexcitable material layer at 600° C. to 850° C.
 13. The methodaccording to claim 11, wherein the annealing comprises annealing thephotoexcitable material layer in an ammonia atmosphere.